potentially reversed OT primaries? Now Express trouble shooting

[aceofbones]

You have your bias terms reversed !

Hmm, pretty sure that's how the voltage chart from the Express build guide lists them. I just went by what it said, my bad.

It looks like a trainwreck with EL34s wants like -33 volts to show up on pin 5 of each output tube to keep them out of burn up city!

I would want to see you have a bias voltage range of -23 to -36 volts.
I would use the alternate 150k as the schematic calls for in the bass circuit to replace the 220k to produce more negative voltage.

I would not put the output tubes back in until yo get that -33 volts on pin 5!

Yeah, I wondered if this was because I've only used up to a 60 watt bulb in my limiter? The bias has increased as I've gone up from 25w through 40w to 60w.

I'll look over the grounds. I thought I had them like a traditional Express but I'll look again.

Thank you.

[sluckey]

None of your voltage measurements will mean anything while you are using that LBL. Since the bulb did not glow brightly it's time to put the LBL under the bench and plug the amp straight into the wall. Then your voltage measurements will be meaningful.

[aceofbones]

for the first look, heater wiring looks sloppy. remove all heater wires from the preamp and redo again, twist the wires properly..

The Express is traditionally this loose loop for the heaters. I printed off a gutshot, to scale, from moderator Colossal's own Express and followed his heater wiring almost exactly. His is a fantastic example of an Express and I know he had no issues with it. I'm not saying you are wrong, only explaining my reasoning for the layout of the heater wiring.

[aceofbones]

None of your voltage measurements will mean anything while you are using that LBL. Since the bulb did not glow brightly it's time to put the LBL under the bench and plug the amp straight into the wall. Then your voltage measurements will be meaningful.

Ok, thanks Sluckey. I guess that will be the first thing I do my next session on the amp.

[bepone]

The Express is traditionally this loose loop for the heaters. I printed off a gutshot, to scale, from moderator Colossal's own Express and followed his heater wiring almost exactly. His is a fantastic example of an Express and I know he had no issues with it. I'm not saying you are wrong, only explaining my reasoning for the layout of the heater wiring.

ok . but i dont like this heater wiring what is consider like standard at all
another suspect - VVR.
this example what youre reffering to, has VVR or not?
can you bypass VVR at the rectifier board, and remove the wires from it totally and startup the amp again?

[Stevem]

Please leave revising your heater wiring for dead last after you have proved out that the rest of the amp functions as it should!
People are side tracking you right now!

[Colossal]

The Express is traditionally this loose loop for the heaters. I printed off a gutshot, to scale, from moderator Colossal's own Express and followed his heater wiring almost exactly. His is a fantastic example of an Express and I know he had no issues with it. I'm not saying you are wrong, only explaining my reasoning for the layout of the heater wiring.

ok . but i dont like this heater wiring what is consider like standard at all
another suspect - VVR.
this example what youre reffering to, has VVR or not?
can you bypass VVR at the rectifier board, and remove the wires from it totally and startup the amp again?

I agree that the original method of wiring the Express' heaters is a little suspect, but nevertheless, it does result in a very quiet amp. On the build Ace is referencing, I copied Ken Fischer's wiring from Francesca.

Ace: do you have a capacitor before the VVR? A VVR can be unstable if there is no "upstream" reservoir capacitance to draw from. The correct installation should be:

Rectifier -> some capacitance -> [ VVR ] -> B+1/Cap -> 1k 25W -> B+2/Cap -> 18k2 -> B+3/Cap etc

[aceofbones]

Ace: do you have a capacitor before the VVR? A VVR can be unstable if there is no "upstream" reservoir capacitance to draw from. The correct installation should be:

Rectifier -> some capacitance -> [ VVR ] -> B+1/Cap -> 1k 25W -> B+2/Cap -> 18k2 -> B+3/Cap etc

Hey Colossal, thanks for the reply. Yes, I believe I do. My wiring is the exact same as the layout I posted, so after the rectifier it goes to c16(40uf) then to the standby switch as normal. When taken off standby it then goes to the VVR then returns to B+1 in the same order you’ve written above.

[Phil_S]

ok . but i dont like this heater wiring what is consider like standard at all

I don't want to be critical, but this recommendation about the heaters isn't right. Please spend some time looking in the Trainwreck Files section of the forum, where you will see Ken Fisher's extensive use of loose loop heater wiring. If it was a problem, he wouldn't have done it that way. You don't have to like what he did, just understand that it's not a problem and and it isn't done wrong.

[aceofbones]

So, I've removed the bulb limiter and powered it up full power. No tubes in for this run. No release of magic smoke, so a decent start.

Bias only ranges from -20.7v to -29.5v so I'll have to address that 220k resistor. I don't have any spare resistors left so will have to order some in.

Now, I understand that without tubes my plate voltages don't mean much, but I was wondering how far down they can come after tubes are installed? Currently the plate voltages of V1-V3 all sit at about 400v. Is something messed up or will installing tubes really bring them down to the expected 182-267v range on the Express voltage chart. Also, should I now go ahead and install V1-V3 tubes, even though I can't install the power tubes until I sort out my bias range?

Mains - 120v
Secondaries - 595v
B+1 402v
B+2 402v
B+3 398v
B+4 398v
B+5 398v

[Stevem]

Yes, without having the high current draw of having the output tubes even in and just idling your plate voltage will be at least 15 volts too high I would say.

If you have a 270k 1/2 watt resistor place that in parallel with the 220k there already and that will get your down to 120k which might still work range adjustment wise.
At any rate it will not be harmful as you will just have more bias voltage then you need and the outputs will just be biased cold.

[sluckey]

should I now go ahead and install V1-V3 tubes, even though I can't install the power tubes until I sort out my bias range?

Yes. Then recheck plate voltages and you'll see them come down considerably.

[aceofbones]

If you have a 270k 1/2 watt resistor place that in parallel with the 220k there already and that will get your down to 120k which might still work range adjustment wise.
At any rate it will not be harmful as you will just have more bias voltage then you need and the outputs will just be biased cold.

I don't, but I do have a 1/2w 1M. Using that in parallel should put me in the 180k range if my math is correct. Otherwise all I have left in 1/2w resistors is a handful of 10k, 33k and a single 100k

[aceofbones]

should I now go ahead and install V1-V3 tubes, even though I can't install the power tubes until I sort out my bias range?

Yes. Then recheck plate voltages and you'll see them come down considerably.

Ok, I've put the preamp tubes in and fired it up. Heaters all lit up. Voltages did drop, not as much as I was anticipating, but I assume the power tubes will cause the largest drop.

With V3 in, the noise has returned, although very, very quiet since there's no power tubes in. This is still with all control pots turned off. If I raise the volume to about 9 o'clock then the noise goes away, anything below 9 and it's there. The noise only starts when I take the amp out of standby, but it will continue to be audible even after I put it back in standby or power off the mains switch. At that point its audible and slowly 'drains' away. I also found if I'm checking voltages on V3 and touch pin 7 that the noise stops immediately.

[Stevem]

Ok
1) use that 1 meg to parallel that 220k.

2) confirm that you can set the bias voltage on pins 5 to -33 volts and leave it set there,

3) hook up a voltmeter set for AC volts across your load resistor.

4) pop in the output tubes and power on, if you read anything above 1 volt on the meter then the amp is oscillating and we need to help you track down where and why!